1. Field of the Invention
This invention relates to an improved catalyst which is suitable for use as a catalyst for auto emission control, especially as a three-way catalyst.
2. Description of the Previously Published Art
Auto emission catalysts have been made from two types of supports. One is a large monolith solid structure which has many channels through it per square inch of cross section. These monoliths are traditionally wash coated with a slip material to provide porosity to increase the surface area. A problem with the monoliths is that they are difficult to replace in a catalytic converter. Furthermore, they are limited in their performance by laminar-flow transport properties.
The other type of support is a particulate material such as a bead or an extrudate. An example of a bead is disclosed in Sanchez U.S. Pat. No. 4,390,456. These beads have been very satisfactory for automobile use. Because of their solid nature and close packing, however, they can present a design problem when making a deep packed catalyst bed. Such a deep bed will exhibit a large pressure drop.
Hollow ceramic pellets for an auto exhaust catalyst support have been suggested by C. B. Lundsager in U.S. Pat. No. 3,907,710. However, the support in the examples was made from cordierite which has a low BET nitrogen surface area. It was thus necessary to coat the cordierite support with an alumina slip which contained a ceria promoter and the catalytic metals such as platinum or palladium. These coated pellets were large with a diameter of 0.25 in. (6.35 mm).
3. Objects of the Invention
It is an object of this invention to provide a doubly promoted catalyst suitable for use as an oxidizing, reducing or three-way catalyst for emission control and especially automotive emission control having good conversion efficiency after a sufficiently long period of aging.
It is a further object of this invention to produce a catalyst suitable for use as an auto emission catalyst where they can be packed in a bed without a large pressure drop across the bed.
It is a further object of this invention to produce a catalyst suitable for use as an auto emission catalyst in the form of a hollow cylindrical extrudate which is internally vaned and where it is not necessary to provide a slip coating on the extruded support.
It is a further object of this invention to produce a catalyst suitable for use as an auto emission catalyst which is made of a transitional alumina extrudate to provide a large internal or BET nitrogen surface area.
It is a further object of this invention to produce a catalyst which is doubly promoted with ceria and an alkali metal to provide good conversion efficiency when used for auto emission control.
It is a further object of this invention to produce a catalyst suitable for use as an auto emission catalyst which has a large geometric surface area per reactor volume.
It is a further object of this invention to convert noxious components of exhaust gas to innocuous entities using the catalyst of this invention.
These and further objects will become apparent as the description of the invention proceeds.